The field of so-called frameless stereotaxy, which includes use of digitized stereotactic navigators of a variety of kinds, has been well known for several years. Many types of stereotactic or so-called frameless digitizers are available, and such stereotactic navigators include mechanical operating arms, ultrasonic localizers, infrared light-tracking digitizers, magnetic digitizers, and others. These digitizers are designed to provide data relative to detection apparatus that can be used to track their position or the position of an instrument in space relative to a surgical field. For example, a surgical instrument mounted with light-emitting diodes or attached to a mechanical operating arm can send electronic signals to a computer graphic workstation which can assimilate these signals and determine the position and movement of the stereotactic navigator. Typically, to relate the position quantitatively of the stereotactic navigator to the patient's anatomy, a calibration maneuver is performed. For example, taking the case of neurosurgery, where the selected target position would be inside the brain, the patient is scanned in an imaging scanner. The image scan data is usually stacks of two-dimensional data that has been referenced into a stereotactic scanner coordinate frame by means of index markers placed on the patient's head or use of natural landmarks that are identifiable both in the image scan data and also physically on the patient's anatomy. The calibration process for the stereotactic navigator might consist of touching the instrument which holds the navigator devices to a series of three or more index marker points located on the surface of the patient's head or even contour tracings or natural anatomical landmarks on the patient's head, and by registering the electronic data in these particular calibration point positions, the stereotactic navigator can be "calibrated" relative to the patient's physical anatomy and therefore relative to the image scanner data representative of the tomographic or three-dimensional image (CT, MRI, PET, SPECT, etc.) of the patient's head stored in a computer graphic workstation. Once the calibration process is complete, then movement of the stereotactic navigator relative to the patient's anatomy will enable visualization on the computer graphic workstation of where the instrument probe is pointing towards the anatomy and inside of the anatomy. In this way, the navigator can be used to plan interventions in the brain or to visualize the internal anatomy of the brain including pathology, pre-operative planning, or intraoperative guidance to neurosurgery. This technique is now known in the state of the art.
One of the problems associated with such stereotactic navigators can occur in the sequence of the operation. For example, if markers are placed on the patient's skin during the image scanning and the patient is then brought to the operating room, he is anesthetized and put into a head clamp. Such a head clamp is shown in FIG. 1. The stereotactic navigator is then calibrated off of the positions of the index markers, and typically there are three or more of these markers located around the patient's head. By the way, these markers could be natural anatomical landmarks or the surface contour of the patient's head, for that matter. Once the calibration of the navigator is complete, the patient's head is sterile draped and the operation can begin. The navigator can then be used to point at the patient's head, and assessment can be made of the appropriate approach, for example, to a target within the head. A problem can occur, however, after sterile draping because the sterile draping will frequently cover up or totally obscure the index markers which enable the calibration in the first place, at a time when the field was not sterilely prepared. After sterile draping and the procedure begins, if for any reason, such as a power failure or movement of the navigator's interrogation system relative to the head, the navigator may be out of calibration relative to the patient's anatomy, and thus the navigator will be out of calibration with the three-dimensional data set from the image scanner. The utility of the navigator is then lost. One example of this might be power failure during the operation which, for the case of the mechanical operating arm, would shut down the readouts for the encoders which read out the position of the articulating links of the arm. Another example might be the movement of the patient's head relative to interrogation cameras in the case of light-emitting diode (LED) tracking systems of an instrument or a microscope. Yet another example of an intraoperative calibration loss would be movement of the relative position of a mechanical operating arm or camera or ultrasonic tracking system devices relative to the patient's head, which, again, would throw off the pre-calibration information relative to the navigator. All of these situations have the very undesirable effect of shutting down the use of the navigator at a time in the operation when it might be crucially needed. After sterile draping, the calibration points, be they natural landmarks or markers on the patient's skin or other localizers, cannot be accessed by the navigator since they are covered by the sterile drape, and therefore a recalibration based on these points is not feasible without breaking sterility. Breaking sterility might be impossible if a surgical opening has already been made.
It has been practice in some use of the Radionics OAS Operating Arm, after a skin incision has been made and the skull exposed, to make small drill holes or divot holes in the patient's skull surrounding the surgical opening or burr hole so that once the draping and sterile field have been achieved, access of these secondary registration or calibration points could be made, their position having been known or determined from the initial calibration step. This type of natural bony landmark recalibration process is possible to "restart" or "recalibrate" the digitizer, however, if the digitizer fails before such divots or bony landmarks can be established for reference, then there is still no way of recalibrating the navigator to the anatomy, and the navigator is of no use further.
Thus, one of the objectives of the present invention is to provide a means of recalibrating a stereotactic navigator in an intraoperative setting, and especially after a sterile draping has been made.
Another object of the present invention is to enable recalibration of a stereotactic digitizer in the event that the patient's anatomy, such as the patient's head with a head clamp on it, has moved relative to the means of tracking the digitizer, for example, moves with respect to the base of an operating arm that has been damped relative to a patient's head clamp or relative to cameras or ultrasonic detectors which are tracking stereotactic digitizers.
Another object of the present invention is to provide a means of recalibrating a stereotactic digitizer should there be a power failure or interruption of the equipment for any reason, especially between the time of initial calibration when the surgical field is unsterile and the time when the surgical field has been sterile draped and a surgical opening made.
Yet another object of the present invention is to provide a recalibration system that can be used with any type of stereotactic navigator, whether it be mechanical arm, LED optical tracking, ultrasonic tracking, magnetic tracking, etc. Another object of the present invention is to provide a recalibration device which can be moved with the patient's head in the case of neurosurgery so that it is always well established in mechanical relationship to the patient's anatomy.
Yet another object of the present invention is to have a recalibration device which can be used unsterile in a given position relative to the patient's anatomy during a calibration process and then, after sterile draping, the device can be autoclaved or otherwise sterilized during the operation and reset back onto the patient's head or head clamping means during the operation in a repeated or repeatable fashion and/or in exactly the same position as it was previously. In this context, an object of the present invention is to have a recalibration device that can be repeatedly relocated relative to a patient immobilization structure in exactly the same position so that it can be removed if it is in the way of the surgery, but can be put back on in the event that intraoperative recalibration is necessary.
The description of the invention which follows shows how these and other objectives can be achieved by it.